Nonaqueous lithium ion batteries have a high energy density compared to conventional aqueous batteries such as Ni—Cd and Ni—H batteries, and can be manufactured in a small size. Therefore, such batteries are widely used in portable devices such as cellular phones and PCs. Moreover, LiCoO2 is generally used as the cathode material of lithium ion batteries which are presently being popularized.
However, several problems have been raised regarding the direct application of LiCoO2 to large batteries used in hybrid vehicles, electric vehicles, and uninterruptible power systems which are expected in the future.
For example, one of the problems raised concerns resources and costs. Since LiCoO2 uses cobalt (Co) which is a rare metal, the use of a large amount of cobalt may cause resource and cost problems.
Moreover, another problem raised is about the danger of explosion. Since LiCoO2 releases oxygen at high temperatures, abnormal heating or short-circuiting of batteries may lead to the danger of explosion. Therefore, it is highly risky to apply LiCoO2 to large batteries.
In this regard, as a substitute for the cathode materials that use LiCoO2, cathode materials which are cheap and less dangerous and which have a phosphate skeleton have been proposed recently. Among them, LiFePO4 having an olivine structure as disclosed in Patent Citation 1 and Non-Patent Citation 1 is attracting worldwide attention as a material satisfying the resource, cost, and safety requirements.
Olivine-based cathode materials expressed by a composition such as LiFePO4 use iron (Fe) as is clear from the composition, and from the resource perspective, iron is abundant in the natural world and cheap compared to cobalt and manganese-based cathode materials. Moreover, since the olivine-based cathode materials have a covalent bond of phosphorous and oxygen, such materials will not release oxygen at high temperatures unlike cobalt-based cathode materials and can be said to be materials having excellent safety properties.
However, although LiFePO4 has the above-mentioned advantages, concerns have been raised about its properties. One problem is low conductivity. However, many reports show that the low conductivity has been solved by recent improvements, particularly by preparing a composite with carbon or coating the surface with carbon.
Another problem is low diffusibility of lithium ions during charge and discharge. In compounds having a layered structure as in LiCoO2 and a spinel structure as in LiMnO2, the diffusion of lithium during charge and discharge takes place in two or three directions. In contrast, in an olivine structure of LiFePO4, the diffusion of lithium takes place in only one direction. In addition, since an electrode reaction during charge and discharge is a 2-phase reaction in which conversion between LiFePO4 and FePO4 occurs repeatedly, LiFePO4 has been considered to be disadvantageous for quick charge and discharge.
A method of reducing the particle size of LiFePO4 particles is considered as the most effective countermeasure.
It is considered to be able to cope with quick charge and discharge if a diffusion distance is decreased by the reduced particle size even when the diffusion takes place in only one direction.
The simplest LiFePO4 synthesis method is a method called a solid-phase method. Briefly, this method is a method of mixing Li, Fe, and P sources in stoichiometric proportions and baking the mixture in an inert atmosphere. This method has a problem in that unless the baking conditions are chosen suitably, it is unable to obtain a resulting material having an intended composition and it is difficult to control a particle size.
Moreover, research has also been made on liquid-phase synthesis using a hydrothermal reaction.
An advantage of the hydrothermal reaction is the ability to obtain a resulting material having a high purity at a temperature which is far lower than a solid-phase reaction. However, in the case of the hydrothermal reaction, particle size control relies greatly on preparation conditions such as a reaction temperature and time. Moreover, even when the particle size is controlled under these preparation conditions, the particle size is often influenced by the performance of a manufacturing apparatus itself, and there is a difficulty in reproducibility.
A means for reducing the particle size through reaction control in the hydrothermal synthesis of LiFePO4-based materials is disclosed in Patent Citation 2 and Non-Patent Citation 2, for example. A method of carrying out a reaction by adding organic acids and ions such as CH3COO−, SO42−, or Cl− to a solvent at the same time and adding an excess of Li to this reaction, thus obtaining single-phase LiFePO4 micro-particles is proposed in Patent Citation 2 and Non-Patent Citation 2.
Moreover, an attempt to obtain LiFePO4 having a small particle size by mechanically grinding a reaction intermediate is disclosed in Patent Citation 3.    Patent Citation 1: JP-B-3484003    Patent Citation 2: JP-A-2008-66019    Patent Citation 3: JP-T-2007-511458    Non-Patent Citation 1: A. K. Padhi et al., J. Electrochem. Soc., 144, 4, 1188 (1997)    Non-Patent Citation 2: Keisuke Shiraishi et al., Journal of the Ceramic Society of Japan, 112, 1305, S58 (2004)